Multiple level programming in a non-volatile memory device

ABSTRACT

The programming method of the present invention minimizes program disturb in a non-volatile memory device by initially programming a lower page of a memory block. The upper page of the memory block is then programmed.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and inparticular the present invention relates to non-volatile memory devices.

BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include personal computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code and system data such as a basic input/output system (BIOS)are typically stored in flash memory devices for use in personalcomputer systems.

As the performance and complexity of electronic systems increase, therequirement for additional memory in a system also increases. However,in order to continue to reduce the costs of the system, the parts countmust be kept to a minimum. This can be accomplished by increasing thememory density of an integrated circuit.

Memory density can be increased by decreasing the distance between thefloating gate cells and by decreasing the size of the cells.Additionally, using multi-level cells (MLC) can increase the amount ofdata stored in an integrated circuit without adding additional cellsand/or increasing the size of the die. The MLC method stores two or moredata bits in each memory cell.

MLC requires tight control of the threshold voltages in order to usemultiple threshold levels per cell. One problem with non-volatile memorycells that are closely spaced, and MLC in particular, is the floatinggate-to-floating gate capacitive coupling causes interference betweencells. The interference shifts the threshold voltage of neighboringcells as one cell is programmed. This is referred to as a programdisturb condition that may program cells that are not desired to beprogrammed.

One way that has been used to reduce this problem is to perform lowerpage (i.e., lower threshold voltage) programming of cells neighboring afirst cell prior to upper page programming of any cell. Thus,interference is reduced since the threshold voltage of the affected cellis going to be re-adjusted when the upper page programming is performed.This programming method, however, introduces complexity into theprogramming process.

For the reasons stated above, and for other reasons stated below thatwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora less complex method for programming multiple level, non-volatilememory cells while reducing program disturb.

SUMMARY

The above-mentioned problems with flash memories and other problems areaddressed by the present invention and will be understood by reading andstudying the following specification.

The present invention encompasses a method for programming a multiplelevel, non-volatile memory device. The memory device has a plurality ofmemory cells that are organized into a plurality of memory blocks. Eachmemory block has bit line columns and word line rows.

The method initially programs a lower page of a first memory block ofthe plurality of memory blocks. The lower page programming starts at alowest word line of the first memory block and continues in anincreasing word line direction until the entire lower page of the firstmemory block is programmed. The upper page of the first memory block isthen programmed. This programming starts at the lowest word line of thefirst memory block and continues in an increasing word line directionuntil the entire upper page of the first memory block is programmed.

Further embodiments of the invention include methods and apparatus ofvarying scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified diagram of one embodiment for a NAND flashmemory array of the present invention.

FIG. 2 shows a diagram of one embodiment of a method of the presentinvention for multiple level programming of a memory device.

FIG. 3 shows a simplified circuit diagram of a non-volatile memory arrayin accordance with the multiple level programming method embodiment ofFIG. 2.

FIG. 4 shows a series of tables of threshold voltages in accordance withthe multiple level programming embodiment of FIG. 2.

FIG. 5 shows a block diagram of one embodiment of an electronic systemof the present invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof.

FIG. 1 illustrates a simplified diagram of one embodiment for asemiconductor NAND flash memory array of the present invention. Thismemory array is for purposes of illustration only as the presentinvention is not limited to NAND flash but can be used for other flasharchitectures (e.g., NOR, AND) and other non-volatile memorytechnologies such as electrically erasable programmable read only memory(EEPROM).

The memory array of FIG. 1, for purposes of clarity, does not show allof the elements typically required in a memory array. For example, onlythree bit lines are shown (BL1, BL2, and BLN) when the number of bitlines required actually depends upon the memory density and chiparchitecture. The bit lines are subsequently referred to as (BL1–BLN).The bit lines (BL1–BLN) are eventually coupled to sense amplifiers (notshown) that detect the state of each cell.

The array is comprised of an array of floating gate cells 101 arrangedin series strings 104, 105. Each of the floating gate cells 101 arecoupled drain to source in each series chain 104, 105. A word line(WL0–WLN) that spans across multiple series strings 104, 105 is coupledto the control gates of every floating gate cell in a row in order tocontrol their operation. In one embodiment, an array is comprised of 32word lines. However, the present invention is not limited to any oneword line quantity.

In operation, the word lines (WL0–WLN) select the individual floatinggate memory cells in the series chain 104, 105 to be written to or readfrom and operate the remaining floating gate memory cells in each seriesstring 104, 105 in a pass through mode. Each series string 104, 105 offloating gate memory cells is coupled to a source line 106 by a sourceselect gate 116, 117 and to an individual bit line (BL1–BLN) by a drainselect gate 112, 113. The source select gates 116, 117 are controlled bya source select gate control line SG(S) 118 coupled to their controlgates. The drain select gates 112, 113 are controlled by a drain selectgate control line SG(D) 114.

In the embodiment of FIG. 1, the top of the array (i.e., WL0) is at thetop of the page and the bottom of the array (i.e., WLN) is at the bottomof the page. However, these labels are for purposes of illustration onlyas WL0 can also begin at the bottom of the page with the word linenumbers increasing towards the top of the page.

Each cell can be programmed as a single bit per cell (i.e., single levelcell—SLC) or multiple bits per cell (i.e., multiple level cell—MLC).Each cell's threshold voltage (V_(t)) determines the data that is storedin the cell. For example, in a single bit per cell, a V_(t) of 0.5Vmight indicate a programmed cell while a V_(t) of −0.5V might indicatean erased cell. The multilevel cell may have multiple V_(t) windows thateach indicate a different state. Multilevel cells take advantage of theanalog nature of a traditional flash cell by assigning a bit pattern toa specific voltage range stored on the cell. This technology permits thestorage of two or more bits per cell, depending on the quantity ofvoltage ranges assigned to the cell.

For example, a cell may be assigned four different voltage ranges of 200mV for each range. Typically, a dead space or margin of 0.2V to 0.4V isbetween each range. If the voltage stored on the cell is within thefirst range, the cell is storing a 01. If the voltage is within thesecond range, the cell is storing a 00. This continues for as manyranges that are used for the cell.

The embodiments of the present invention are not limited to two bits percell. Some embodiments may store more than two bits per cell, dependingon the quantity of different voltage ranges that can be differentiatedon the cell.

During a typical prior art programming operation, the selected word linefor the flash memory cell to be programmed is biased with a programmingpulse at a voltage that, in one embodiment, is greater than 16V. Averification operation with a word line voltage of 0V is then performedto determine if the floating gate is at the proper voltage (e.g., 0.5V).The unselected word lines for the remaining cells are typically biasedwith at approximately 10V during the program operation. In oneembodiment, the unselected word line voltages can be any voltage aboveground potential. Each of the memory cells is programmed in asubstantially similar fashion.

A typical memory block may be comprised of 64 pages of single levelcells. An MLC memory block is typically comprised of 128 pages. When oneof these pages is accessed, the remaining pages in the block canexperience a disturb condition. This occurs for both a read and a writeaccess. In both cases, the pages share common word lines and bitlinesthat can experience higher programming/read voltages whenever any one ofthe pages is programmed/read. These voltages can cause problems bydisturbing the distributions for the cells that are not being accessed.

FIG. 2 illustrates a diagram of one embodiment of a method of thepresent invention for multiple level programming of a memory device.This diagram shows the order in which cells and pages of the device areprogrammed. For purposes of clarity, only eight cells (i.e., four wordlines) are discussed. However, this method is repeated to include anentire block of memory word lines. This concept is illustrated anddiscussed subsequently with reference to FIG. 3.

Each row of the diagram of FIG. 2 illustrates the write operation toeach of cells 1–8 that are illustrated in FIG. 1. Each column 200, 201illustrates the page being programmed (i.e., first or second). Withineach page 200, 201 are the circled numbers indicating the order of thewrite operation.

In a first write operation, one bit of data is written to the first pageof memory cell 1. A second write operation writes one bit of data to thefirst page (i.e., lower page) of memory cell 2. A third write operationwrites one bit of data to the first page of memory cell 3. A fourthwrite operation writes one bit of data to the first page of memory cell4. A fifth write operation writes one bit of data to the first page ofmemory cell 5. A sixth write operation writes one bit of data to thefirst page of memory cell 6. A seventh write operation writes one bit ofdata to the first page of memory cell 7. An eighth write operationwrites one bit of data to the first page of memory cell 8.

Similarly, a ninth write operation writes one bit of data to the secondpage (i.e., upper page) of memory cell 1. A tenth write operation writesone bit of data to the second page of memory cell 2. An eleventh writeoperation writes one bit of data to the second page of memory cell 3. Atwelfth write operation writes one bit of data to the second page ofmemory cell 4. A thirteenth write operation writes one bit of data tothe second page of memory cell 5. A fourteenth write operation writesone bit of data to the second page of memory cell 6. A fifteenth writeoperation writes one bit of data to the second page of memory cell 7. Asixteenth write operation writes one bit of data to the second page ofmemory cell 8.

As is well known in the art, an erase operation is performed on thememory block prior to the above-described write operations. Thisinitializes the memory cells to the logical “11” state. Additionally, averify operation is performed after the write operations to verifyproper programming. The erase and verify operations are well known inthe art and not discussed further.

FIG. 3 illustrates a write pattern to a memory array using the multiplelevel programming embodiment of FIG. 2. For purposes of clarity, only asmall portion of the cells of a memory block are shown.

Each cell shows a pair of numbers near each memory cell gate thatindicate the write operation order as discussed previously withreference to FIG. 2. The lower number indicates the order of programmingthe lower page of the cell. The upper number indicates the order ofprogramming the upper page of the cell. This pattern is repeatedthroughout the block of 128 pages.

FIG. 4 illustrates a series of tables of threshold voltages (V_(t))400–402 resulting from the programming method of the present invention.The first table 400 illustrates the threshold voltages of a matrix ofnine memory cells in an erased (i.e., logical “11”) state. In thisembodiment, each of the threshold voltages are −3.50V in this state.

The second table 401 illustrates the resulting threshold voltages of thememory cell matrix after the lower page programming operation. Thememory cells with the 1.00V threshold voltages have been programmed to alogical “01” state, while the remaining cells are still in the erasedstate. It can be seen that the cells adjacent to the programmed cellshave exhibited a slight change in their threshold voltages.

The third table 402 illustrates the “worst case” resulting thresholdvoltages of the memory cell matrix after all of the cells of the matrixhave been programmed to a logical “01” state. The cells that have athreshold voltage of 1.15V exhibit a slight amount of programcapacitance interference since they would be at 1.00V withoutexperiencing interference from neighboring cells. The center cell 410 ofthe matrix 402 is surrounded by cells that have been programmed and,therefore, should exhibit the most program disturb. However, this cellonly experiences a disturb condition of 150 mV. In a typical prior artprogram operation, such a cell might have a threshold voltage that is230 mV from the programmed threshold voltage.

FIG. 5 illustrates a functional block diagram of a memory device 500that can incorporate the flash memory array and programming methodembodiments of the present invention. The memory device 500 is coupledto a processor 510. The processor 510 may be a microprocessor or someother type of controlling circuitry. The memory device 500 and theprocessor 510 form part of an electronic system 520. The memory device500 has been simplified to focus on features of the memory that arehelpful in understanding the present invention.

The memory device includes an array of flash memory cells 530 asdescribed above with reference to FIG. 1. The memory array 530 isarranged in banks of rows and columns. The control gates of each row ofmemory cells is coupled with a word line while the drain and sourceconnections of the memory cells are coupled to bitlines. As is wellknown in the art, the connections of the cells to the bitlinesdetermines whether the array is a NAND architecture, an ANDarchitecture, or a NOR architecture.

An address buffer circuit 540 is provided to latch address signalsprovided on address input connections A0–Ax 542. Address signals arereceived and decoded by a row decoder 544 and a column decoder 546 toaccess the memory array 530. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depends on the density and architecture of thememory array 530. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 500 reads data in the memory array 530 by sensingvoltage or current changes in the memory array columns usingsense/buffer circuitry 550. The sense/buffer circuitry, in oneembodiment, is coupled to read and latch a row of data from the memoryarray 530. Data input and output buffer circuitry 560 is included forbi-directional data communication over a plurality of data connections562 with the controller 510. Write circuitry 555 is provided to writedata to the memory array.

Control circuitry 570 decodes signals provided on control connections572 from the processor 510. These signals are used to control theoperations on the memory array 530, including data read, data write(program), and erase operations. The control circuitry 570 may be astate machine, a sequencer, or some other type of controller. In oneembodiment, the control circuitry 570 is responsible for executing theembodiments of the programming method of the present invention forimproved interference immunity in a MLC array.

The flash memory device illustrated in FIG. 5 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of flashmemories are known to those skilled in the art.

CONCLUSION

In summary, the embodiments of the present invention improves marginsbetween MLC levels while maintaining programming throughput. This isaccomplished by initially programming all of the lower page bits of amemory block. The upper page bits of the memory block are thenprogrammed. In one embodiment, this programming method can reduceinterference by 30 –40% in a programmed cell as compared to prior artmethods of programming.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A method for programming a multiple level, non-volatile memory devicehaving a plurality of memory blocks, each memory block comprising aplurality of memory cells each having an upper page of data and a lowerpage of data, the method comprising: initially programming the lowerpage of each of the plurality of memory cells of a first memory blockprior to programming the upper page of the plurality of memory cells ofthe first memory block; and programming the upper page of the pluralityof memory cells of the first memory block only after the lower page isprogrammed.
 2. The method of claim 1 wherein the first word line is partof a memory block comprising 128 pages.
 3. The method of claim 1 whereineach of the plurality of memory cells is adapted to store a plurality ofbits.
 4. The method of claim 1 wherein the memory device is arranged ina NAND architecture.
 5. The method of claim 1 wherein the memory deviceis arranged in a NOR architecture.
 6. The method of claim 1 and furtherincluding the plurality of memory cells being programmed from an erasedstate.
 7. The method of claim 6 wherein the erased state is a logical“11” state.
 8. A method for programming a multiple level flash memorydevice having a plurality of memory cells organized into a plurality ofmemory blocks, each memory cell having a lower page of data and an upperpage of data, the method comprising: programming a lower page of a firstmemory block of the plurality of memory blocks such that the programmingstarts at a first word line closest to a source line, the programmingcomprising substantially simultaneously programming alternate cells in abit line direction wherein all of the lower page of data of the firstmemory block is programmed prior to programming the upper page; andprogramming an upper page of the first memory block, after the lowerpage is programmed, starting at the first word line, the programmingcomprising substantially simultaneously programming alternate cells inthe bit line direction.
 9. The method of claim 8 wherein programming thelower page of the first memory block includes performing a first writeoperation on each alternate memory cell of the first word line in thebit line direction, performing a second write operation on the remainingmemory cells of the first word line, performing a third write operationon each alternate memory cell of a second word line in the bit linedirection, performing a fourth write operation on the remaining memorycells of the second word line, performing a fifth write operation oneach alternate memory cell of a third word line in the bit linedirection, performing a sixth write operation on the remaining memorycells of the third word line, performing a seventh write operation oneach alternate memory cell of a fourth word line in the bit linedirection, and performing an eighth write operation on the remainingmemory cells of the fourth word line.
 10. The method of claim 9 whereinprogramming the upper page of the first memory block includes performinga ninth write operation on each alternate memory cell of the first wordline in the bit line direction, performing a tenth write operation onthe remaining memory cells of the first word line, performing aneleventh write operation on each alternate memory cell of the secondword line in the bit line direction, performing a twelfth writeoperation on the remaining memory cells of the second word line,performing a thirteenth write operation on each alternate memory cell ofthe third word line in the bit line direction, performing a fourteenthwrite operation on the remaining memory cells of the third word line,performing a fifteenth write operation on each alternate memory cell ofthe fourth word line in the bit line direction, and performing asixteenth write operation on the remaining memory cells of the fourthword line.
 11. The method of claim 8 and further including erasing thefirst memory block prior to programming the first page.
 12. The methodof claim 8 and further including verifying the first memory block afterprogramming the lower and upper pages.
 13. The method of claim 8 whereineach write operation increases a threshold voltage for the programmedmemory cell.
 14. A flash memory device comprising: a memory arraycomprising a plurality of multiple level memory cells, the plurality ofmemory cells organized into memory blocks having a plurality of rows ofcells coupled to word lines and a plurality of columns of cells coupledto bit lines, each memory cell having an upper page of data and a lowerpage of data; and control circuitry coupled to the memory array, thecontrol circuit adapted to program the plurality of memory cells byinitially programming only the lower page of first memory block prior toprogramming the upper page of the first memory block.
 15. The flashmemory device of claim 14 wherein control circuitry is further adaptedto start programming at a word line closest to a source line of thememory array.
 16. The flash memory device of claim 14 wherein thecontrol circuitry is further adapted to perform erase, read, and verifyoperations on the memory array in response to received commands.
 17. Theflash memory device of claim 14 and further including sense amplifierscoupled to the memory array for determining a programmed state of thememory blocks.
 18. The flash memory device of claim 14 wherein theplurality of multiple level memory cells are organized in one of a NANDarchitecture, an AND architecture, or a NOR architecture.